To detect an acoustic signal, distributed acoustic sensing is commonly and effectively used. This method employs fibre optic cables to provide distributed acoustic sensing whereby the fibre optic cable acts as a string of discrete acoustic sensors, and an optoelectronic device measures and processes the returning signal. The operations of such a device is described next.
A pulse of light is sent into the optical fibre, and a small amount of light is naturally back scattered, along the length of the fibre by Rayleigh, Brilliouin and Raman scattering mechanisms. The scattered light is captured by the fibre and carried back towards the source where the returning signal is measured against time, allowing measurements in the amplitude, frequency and phase of the scattered light to be determined. If an acoustic wave is incident upon the cable, the glass structure of the optical fibre is caused to contract and expand within the vibro-acoustic field, consequently varying the optical path lengths between the back scattered light scattered from different locations along the fibre. This variation in path length is measured as a relative phase change, allowing the optical phase angle data to be used to measure the position of the acoustic signal at the point at which light is reflected. The returning signal can also be processed in order to determine the frequency of oscillation of vibration in the structure.
In known distributed acoustic sensing systems (DAS), standard fibre optic cables are utilised to obtain a measurement profile from along the entire length of the fibre at intervals ranging from 1-10 meters. Further details regarding the operation of a suitable DAS system, such as the iDAS™, available from Silixa Limited, of Elstree, UK are given in WO2010/0136809. Systems such as these are able to digitally record acoustic fields at every interval location along an optical fibre at frequencies up to 100 kHz. Since the position of the acoustic sensors is known (the fibre deployment being known), the source of any acoustic signal can be thus identified by means of time-of-arrival calculations. In a typical deployment, the sensing points are usually created by clamps which are used to secure the fibre optic cable to the structure or area it is monitoring.
By way of example, FIG. 1 shows a common arrangement of a known fibre optic cable 1, comprising at least one optical fibre, contained in a series of concentric tubular structures. The cable generally comprises firstly an inner tubular structure, typically called a fibre-in-metal-tube (FIMT) 2, which provides a way of encapsulating very long lengths of optical fibres 5 within a hermetically sealed tube 4. A general construction of a FIMT 2 includes at least one optical fibre 5 encapsulated in a metal tube 4. Additionally, it is common to fill this metal tube 4 with a thixotropic gel 6 in order to protect the optical fibres 5 from environmental disturbances, prevent damage from micro-bending conditions and to help minimise the forces applied during spooling and deployment of the cable. Most importantly for distributed acoustic sensing, the thixotropic gel 6 supports the optical fibre 5, preventing excessive movement within the metal tube 4 which reduces the amount of resonant frequencies. The FIMT 2 is typically then encapsulated by a further outer tube 3, usually containing a filler material.
The optical fibres 5 are typically made of flexible, transparent fibres of glass. The filler material 3 surrounding the FIMT 2 has a lower refractive index than the optical fibres 5 such that light which has been focused into the optical fibres 5 is confined due to total internal reflection, hence enabling the light to propagate down the length of the optical fibres 5 without any light being lost.
There are many applications for which distributed acoustic sensing may be used, for example, monitoring hydraulic fracturing of oil or gas structures and surveillance methods of assets such as oil or gas pipelines and airport runways. In order to monitor such assets, the fibre optic cables are usually secured to the structure or area by clamps distributed along the length of the fibre optic cable. By way of example, FIG. 2 illustrates how fibre optic cables 1 may be used to monitor structures or areas using distributed acoustic sensing.
FIG. 2 shows a fibre optic cable 1 being used to monitor a pipeline 7 that has been deployed underground 9. The fibre optic cable 1 is positioned to run parallel alongside the pipeline 7 and is secured by a series of clamps 8, which are distributed along the length of the pipeline 7. These clamps 8 allow the fibre optic cable 1 to monitor the pipeline 7 through distributed acoustic sensing since the clamps 8 themselves act as an array of acoustic coupling regions. The clamps 8 transmit any vibrations in the pipeline 7, such that the acoustic energy is transmitted to the optical fibres 5.
The clamps are spaced along the fibre at a distance at least equal to or greater than the sensing resolution of the distributed acoustic sensing, typically 1-5 meters. This provides discrete sensing points along the fibre matched to the sensing resolution and prevents any anti-aliasing effects in the detected acoustic signal.
In some deployments, however, it is not possible to secure the cable with clamps, and instead the cable may be inserted in a concrete trench or the like running parallel to a pipe, well, or any other structure being monitored. In this case there are no discrete sensing points as is provided by the clamps, and hence the fibre can sense at all points along its length.
As a consequence, due to the sensing resolution of the fibre being less than the actual resolution of the points at which acoustic energy is being sensed, aliasing effects can occur in the signal, due to undersampling.
Another problem faced when using fibre optic cables in distributed acoustic sensing is that acoustic signals incoming from one direction may overcome acoustic signals incoming from another direction, making it difficult for the fibre optic cable to detect the latter. This may prove problematic for certain applications of distributed acoustic sensing. Consider, by way of example, fibre optic cables used for surveillance of an asset. Acoustic signals emitted by the asset itself may obscure any acoustic signals incoming from the surroundings towards the asset. However, in security surveillance, it is the incoming acoustic signals caused by disturbances in regions surrounding the asset that are of interest. Therefore, it would be advantageous if the fibre optic cable was more acoustically sensitive in the directions corresponding to the surrounding area such that the ability to detect acoustic signals in these direction is greater.